Steering system connections in a conventional automotive vehicle are typically provided via direct mechanical linkages between a steering device and the front wheels of the vehicle. For example, in a rack and pinion steering system the rotation of a steering wheel rotates a pinion gear that is engaged with mating notches or teeth in a rack portion of the steering system. Tie rods connect the rack portion to the wheels, so that any rotational motion of the steering wheel ultimately rotates the road wheels to a resultant steering angle, which may vary depending on the steering ratio provided by the steering system. Other steering linkage designs may be used instead of a rack and pinion design, such as worm gears used in a re-circulating ball steering system. In either example, however, all control linkages are purely mechanical in nature.
By way of contrast, in a by-wire steering system, an electronically controlled steering actuator is positioned on or in proximity to the front drive axle, and one or more transducers and/or other sensors measure or detect the steering request or input to a steering device, which is usually a circular steering wheel. The sensors can be configured to measure a steering angle at the steering wheel, and/or as torque sensors for measuring a rotational force or torque applied to the steering wheel. The electronic input signals representing these measured or detected values are then transmitted electrically to a steering actuator, which executes a steering maneuver in response to the electronic input signals.
By-wire or electronic steering systems may be configured as a full by-wire system, in which a driver is completely decoupled from the road wheels and all steering input signals are electrically transmitted to the steering actuator. Alternately, a by-wire steering system can be configured as a limited by-wire system, in which a conventional mechanical steering linkage is retained, and in which an electronic steering signal is selectively used to augment or modify a steering response in the steering system.
Limited by-wire systems in particular can be configured to selectively vary a gear ratio to a degree that is determined in part by the speed of the vehicle, in order to optimize the steering response. For example, when driving in a parking lot under low speeds, a steering controller may decrease a steering ratio to allow for a tighter or more effortless steering maneuver. The same steering system may then increase a steering ratio when the vehicle operates at highway speeds to produce a desired steering response. However, conventional gear ratio variation methods may be less than optimal under certain dynamic conditions.